Mauro recently wrote down some random thoughts on WSS 3.0/MOSS 2007 - I'm going to quote a piece which I think was very important :
Speaking of upgrade, I think there are two types: Technical and Business. I've seen a lot written already about the various options for upgrading (in-place, side-by-side, etc.). These are all METHODS of the Technical Upgrade. The goal is to simply stand a V3 instance of a V2 portal. This is very different than a Business Upgrade. Here we focus on leveraging new features, redesigning outdated taxonomies, etc. When I present, I explain these (I'm a visual person) as two steps: (1) Lateral (my hands move left to right) to explain a technical upgrade and (2) Upward (my hands my up) to explain a business upgrade. Both are different; each requires its own planning.